Process for preparing an oxidation catalyst composition

ABSTRACT

A process for preparing an oxidation catalyst composition comprises mixing at least one crystalline composite oxide, as the first component, selected from the group consisting of (i) a crystalline composite oxide containing vanadium and phosphorus and showing the characteristic X-ray diffraction peaks as identified in the following Table A and (ii) a crystalline composite oxide containing vanadium and phosphorus and showing the characteristic X-ray diffraction peaks as identified in the following Table B, an aqueous solution, as the second component, containing vanadium and phosphorus, and silica sol, as the third component, to form an aqueous slurry, spray-drying the slurry, and calcining the solid particles thereby obtained: 
     
                       TABLE A                                                     
 
    
     ______________________________________                                    
X-ray diffraction peaks                                                   
(Anticathode: Cu--K.sub.α)                                          
2θ (±0.2°)                                                
______________________________________                                    
15.7°                                                              
19.6°                                                              
24.2°                                                              
27.1°                                                              
28.8°                                                              
30.4°                                                              
______________________________________                                    
 
     
                       TABLE B                                                     
 
    
     ______________________________________                                    
X-ray diffraction peaks                                                   
(Anticathode: Cu--K.sub.α)                                          
2θ (±0.2°)                                                
______________________________________                                    
14.2°                                                              
15.7°                                                              
18.5°                                                              
23.0°                                                              
28.4°                                                              
30.0°                                                              
33.7°                                                              
36.8°                                                              
______________________________________

The present invention relates to a process for preparing an oxidationcatalyst composition. More particularly, the present invention relatesto a process for preparing an oxidation catalyst composition suitablefor use as a catalyst for the production of maleic anhydride byvapour-phase oxidation of a hydrocarbon having at least 4 carbon atoms,especially butane.

It is known that in the process for producing maleic anhydride byvapour-phase oxidation of a hydrocarbon having 4 carbon atoms such asn-butane, n-butenes or butadiene, a composite oxide containing vanadiumand phosphorus as the essential components is effective as a catalyst(U.S. Pat. No. 3,293,268). It is further reported that in this catalyst,crystalline vanadyl phosphate ((VO)₂ P₂ O₇) is effective as the activecomponent (E. Bordes, P Courtine, J. Catal., 57, 236 (1979)). Thecrystal phase of this compound can be identified by the characteristicX-ray diffraction pattern as shown in the following Table B:

                  TABLE B                                                         ______________________________________                                        X-ray diffraction of (VO).sub.2 P.sub.2 O.sub.7                               (Anticathode Cu--K.sub.α ), Major peaks                                        2θ (±0.2°)                                                            Intensity                                                      ______________________________________                                               14.2°                                                                          20                                                                    15.7°                                                                          20                                                                    18.5°                                                                          20                                                                    23.0°                                                                          100                                                                   28.4°                                                                          90                                                                    30.0°                                                                          50                                                                    33.7°                                                                          40                                                                    36.8°                                                                          40                                                             ______________________________________                                    

The present inventors have confirmed that when used as a catalyst forvapour-phase oxidation of n-butane, n-butenes and the like, the crystalphase of the compound exhibits substantially higher catalytic activitiesthan those of the amorphous composite oxide catalysts prepared by theconventional processes and particularly when it is used for theoxidation of butane, the reaction proceeds even at a low temperature ofabout 100° C. Accordingly, it is preferred for the process to use thecatalytically active components having the X-ray diffraction peaks asidentified in the above Table B.

On the other hand, the vapour-phase oxidation reaction for the formationof maleic anhydride from hydrocarbons having 4 carbon atoms includingthe side reation for complete oxidation (i.e. the formation of carbonmonoxide and carbon dioxide) is a highly exothermic reaction.Accordingly, from a view-point of energy efficiency as well as from aview-point of a relatively low limiting concentration for the explosionof the hydrocarbon material relative to air, a fluidized bed catalyticoxidation reaction has been believed to be most suitable for thereaction. As a catalyst developed for this purpose, there is a catalystwhich has been prepared by spray-drying a mixture comprising a vanadyloxalate solution, phosphoric acid, silica sol and a suitableactivity-promoting ingredient (British Pat. No. 1,285,075). The catalystthus obtained is effective for the oxidation of butenes and butadiene.However, for the oxidation of butane, its catalytic activity is not highenough and it is usually required that the reaction be conducted at atemperature of at least 500° C.

There have been several reports relating to fluidized bed catalystsuseful for the oxidation of butane. For instance, Japanese UnexaminedPatent Publication No. 126587/1974 discloses a case where a compositeoxide is formed by contacting a pentavalent vanadium compound with atrivalent phosphorus compound, and then the composite oxide ispulverized into fine powder, which is then used for a fluidized bedreaction. In this process, a crystalline active component can be takenout and the catalytic activity can be adequately improved. However, thestrength and the flowability of the catalyst are not adequate.

Certain possibilities of the pulverized catalyst fluidized bed reactionin which such fine powder of a catalytically active component is used,are also pointed out in Japanese Unexamined Patent Publications No.8788/1975 and No. 33038/1981, and a possibility for the production of afluidized bed catalyst by depositing a complex oxide on a carrier ispointed out in Japanese Unexamined Patent Publication No. 65635/1981.

The present inventors have conducted extensive researches to develop acatalyst useful particularly for the vapour-phase oxidation of n-butaneby a fluidized bed and as a result have found that a catalyst havingsuperior strength and flowability can be prepared by mixing a specialcrystalline oxide, as the first component, containing vanadium andphosphorus, an aqueous solution, as the second component, containingvanadium and phosphorus, silica sol as the third component, to prepare aslurry and spray-drying the slurry. The present invention has beenaccomplished based on this discovery.

It is an object of the present invention to provide a process forpreparing an oxidation catalyst composition suitable for use in theproduction of maleic anhydride by vapour-phase oxidation of ahydrocarbon having at least 4 carbon atoms.

Another object of the present invention is to provide a process forproducing an oxidation catalyst composition suitable for use in theproduction of maleic anhydride by vapour-phase oxidation of n-butane andn-butenes.

A further object of the present invention is to provide a process forpreparing an oxidation catalyst composition for a fluidized bed whichhas superior strength and flowability and which is suitable for use inan industrial production of maleic anhydride by vapour-phase oxidationof n-butane and n-butenes by means of a fluidized bed reactor.

The present invention provides a process for preparing an oxidationcatalyst composition which comprises mixing at least one crystallinecomposite oxide, as the first component, selected from the groupconsisting of (i) a crystalline composite oxide containing vanadium andphosphorus and showing the characteristic X-ray diffraction peaks asidentified in the following Table A and (ii) a crystalline compositeoxide containing vanadium and phosphorus and showing the characteristicX-ray diffraction peaks as identified in the following Table B, anaqueous solution, as the second component, containing vanadium andphosphorus, and silica sol, as the third component, to form an aqueousslurry, spray-drying the slurry, and calcining the solid particlesthereby obtained:

                  TABLE A                                                         ______________________________________                                        X-ray diffraction peaks                                                       (Anticathode: Cu--K.sub.α)                                              2θ (±0.2°)                                                    ______________________________________                                        15.7°                                                                  19.6°                                                                  24.2°                                                                  27.1°                                                                  28.8°                                                                  30.4°                                                                  ______________________________________                                    

                  TABLE B                                                         ______________________________________                                        X-ray diffraction peaks                                                       (Anticathode: Cu--K.sub.α)                                              2θ (±0.2°)                                                    ______________________________________                                        14.2°                                                                  15.7°                                                                  18.5°                                                                  23.0°                                                                  28.4°                                                                  30.0°                                                                  33.7°                                                                  36.8°                                                                  ______________________________________                                    

Now, the present invention will be described in detail with reference tothe preferred embodiments.

In the present invention, a crystalline vanadium-phosphorus compositeoxide showing the characteristic X-ray diffraction peaks as identifiedin Table A or B is used as the first component. As mentioned above, thevanadium-phosphorus composite oxide showing the characteristic X-raydiffraction peaks as identified in Table B is known and it is usuallyprepared by firstly preparing its precursor i.e. a crystalline compositeoxide containing tetravalent vanadium and pentavalent phosphorus andshowing the characteristic X-ray diffraction peaks as identified in thefollowing Table A (hereinafter referred to as "a precursory oxide") andthen calcining the precursory oxide. (The vanadium-phosphorus compositeoxide showing the characteristic X-ray diffraction peaks as shown inTable B will be hereinafter referred to as "a calcined oxide".)

                  TABLE A                                                         ______________________________________                                        X-ray diffraction peaks (Anticathode Cu--K.sub.α)                              2θ (±0.2°)                                                            Intensity                                                      ______________________________________                                               15.7°                                                                          100                                                                   19.6°                                                                          50                                                                    24.2°                                                                          40                                                                    27.1°                                                                          45                                                                    28.8°                                                                          25                                                                    30.4°                                                                          80                                                             ______________________________________                                    

This precursory oxide is known and it can be prepared by the followingknown methods.

(1) In a non-oxidizing acidic solution such as a hydrochloric acidsolution, a pentavalent vanadium such as vanadium pentoxide is dissolvedand reduced, the reduction being completed by a reducing agent such asoxalic acid to obtain a solution containing tetravalent vanadium ions,the solution is reacted with phosphoric acid and a solublevanadium-phosphorus composite thereby formed is precipitated by anaddition of water (Japanese Unexamined Patent Publication No.95990/1976).

(2) A pentavalent vanadium compound such as vanadium pentoxide andphosphoric acid are reacted in an aqueous medium in the presence of areducing agent such as hydrazine hydrochloride or a hydroxylaminehydrochloride, followed by concentration or evaporation to obtaincrystals (Japanese Unexamined Patent Publication No. 45815/1981).

(3) Vanadium pentoxide is reduced in an organic medium such as ethanol,isopropanol or glycerol and then reacted with anhydrous phosphoric acid,and water is azeotropically removed with use of a solvent such asbenzene, whereby crystals are precipitated (U.S. Pat. No. 4,283,288).

Further, some of the present inventors have proposed a method forpreparing the precursory oxide showing the characteristic X-raydiffraction peaks as identified in the above Table A by subjecting anaqueous solution containing pentavalent phosphorus and tetravalentvanadium to hydrothermal treatment at a temperature of from 110° to 250°C. (Japanese patent application No. 32110/1982). In this method, firstlya pentavalent vanadium compound such as vanadium pentoxide is reactedwith phosphoric acid in an aqueous acidic medium containing ahalogen-free reducing agent such as hydrazine hydrate to obtain anaqueous solution containing tetravalent vanadium and phosphoric acid asmajor components and then the aqueous solution is subjected tohydrothermal treatment in a closed container at a temperature of from110° to 250° C., preferably from 120° to 180° C. for about 0.5 to 200hours. As the aqueous medium, water is usually used. A hydrophilicorganic solvent such as an alcohol, a carboxylic acid, an ether or aketone may optionally be combined with water. However, the reducing rateof vanadium is thereby reduced, and accordingly, the amount of such ahydrophilic organic solvent should be at most 50% by weight. Thephosphoric acid concentration in the aqueous medium is usually from 5 to50% by weight, preferably from 5 to 35% by weight. If the phosphoricacid concentration is too high, it is possible that vanadium pentoxidereacts with phosphoric acid before it is reduced and the viscosity ofthe solution tends to be extremely high, whereby the handling becomesdifficult. The reduction can adequately be done with use of the reducingagent in a stoichiometric amount required for reducing the pentavalentvanadium to the tetravalent vanadium, and the reducing agent is usuallyused in an amount within a range of from 95 to 120% of thestoichiometric amount. As the reducing agent, a non-halogen inorganicreducing agent such as hydrazine, hydroxylamine or a phosphate thereofis preferably used. If desired, an organic reducing agent such as oxalicacid may be used, but the use of such an organic reducing agent is notindustrially advantageous. The reduction of vanadium is conductedpreferably by a method which comprises adding vanadium pentoxide to anaqueous solution which has previously been prepared by dissolvingphosphoric acid and the reducing agent, whereby it is possible to formcrystals having higher purity. At the time of the hydrothermaltreatment, it is preferred to add a small amount of finely pulverizedseed crystals to the aqueous solution. By such hydrothermal treatment, aslurry containing grayish blue fine crystals is obtained. These crystalsare the desired precursory oxide, and they are obtainable by evaporatingor spray-drying the slurry or directly from the slurry by solid-liquidseparation such as filtration. By this method, it is possible to obtainthe oxide having finer particle sizes than those obtainable by theabove-mentioned conventional methods.

The precursory oxide obtainable by the foregoing methods may berepresented by the formula (V₂ O₄)(P₂ O₅)(2H₂ O). Thus, the atomic ratioof phosphorus to vanadium (P/V) is theoretically 1.0. Practically,however, when any one of the above-mentioned methods is used, thevanadium compound and the phosphorus compound are preferably reacted ina P/V atomic ratio within a range of from 0.8 to 1.25.

Further, in the first component to be used for the process of thepresent invention, a part of the vanadium atoms may be substituted byvarious metal atoms having an ion radius not very much different fromthe vanadium ions. As such metals, there may be mentioned iron,chromium, aluminum, titanium, cobalt, magnesium, manganese and nickel.When used for a catalyst, the composite oxide partly substituted by suchmetal atoms brings about a remarkable improvement in the catalyticactivity and the stability of the catalytic activity. The proportion ofthe substitution is optionally selected within a range of from 0.005 to0.4 mol, preferably from 0.05 to 0.2 mol of the metal atoms per mol ofvanadium atoms. As a method for introducing such other metal atoms tothe composite oxide, there may be mentioned a method wherein these metalions are added in the form of an inorganic salts such as a chloride, ahydroxide, a sulfate, a nitrate or a carbonate, or an organic salt suchas an oxalate during the process for preparing the precursory oxide.

The X-ray diffraction pattern of the substituted solid solution-typeprecursory oxide thus obtained is more or less shifted from the peaksshown in Table A. However, degree of the shift is within a range of±0.2°.

The precursory oxide is then calcined in an atmosphere of an inert gassuch as argon or nitrogen, whereby the above-mentioned calcined oxide isobtained. In the calcined oxide, vanadium is present substantially inits tetravalent state. It is preferred to calcine it in air to convert apart of vanadium to its pentavalent state. The ratio of the pentavalentvanadium to the total vanadium in the calcined oxide is inter-relatedwith the properties of the catalysts finally obtained. In general, thebest results are obtainable when the ratio of the pentavalent vanadiumis within a range of from 15 to 25%. The properties will be degraded ifthe ratio is greater or smaller than this range. And, if the ratio ofthe pentavalent vanadium exceeds 35%, the crystal phase showing theabove characteristic X-ray diffraction peaks (i.e. the crystal phase of(VO)₂ P₂ O₇) tends to undergo decomposition. Such an excessivelyoxidized calcined oxide has a disadvantage that it sometimes forms arubber-like solid substance during the preparation of the catalyst andthe catalyst thereby obtained tends to have poor properties.Accordingly, the calcination in air should be conducted within a rangewhere the ratio of the pentavalent vanadium is at most 35%. Thepreferred ratio of the pentavalent vanadium in the calcined oxide isfrom 5 to 35%.

Instead of the two step calcination with the inert gas and air, it ispossible to calcine the above-mentioned precursory oxide in air dilutedwith an inert gas to obtain a calcined oxide containing the pentavalentvanadium in a predetermined ratio. In the case where the calcined oxideis prepared by calcining the precursory oxide in ordinary air, it isnecessary to take a precaution not to excessively oxidize the vanadium,for instance, by controlling the temperature.

The calcination of the precursory oxide may be conducted in a furnace ofany optional type. However, it is usual to use a muffle furnace, arotary kiln or a fluidized bed calcination furnace. The calcinationtemperature is usually higher than the dehydration temperature of theprecursory oxide i.e. from 430° to 700° C., preferably from 450° to 600°C.

As mentioned above, the precursory oxide is separated from a slurrycontaining it by means of solid-liquid separation such as filtration orevaporation or spray-drying, and if required, it is further calcined toform a calcined oxide before using it as the first component. Further,in the case where the above-mentioned hydrothermal treatment is used, itis not indispensable to separate the precursory oxide from the slurryand it may be used by itself as the first component. In this case, it ispossible that phosphoric acid, a reducing agent and vanadium pentoxideare added and dissolved in the slurry and the liquid phase therebyobtained is used as the second component. Further, it is also possiblethat phosphoric acid, a reducing agent and vanadium pentoxide are addedand dissolved in a liquid phase obtained by solid-liquid separation ofthe slurry after the hydrothermal treatment and the solution therebyobtained is used as the second component.

As mentioned above, the first component may be used in the form ofeither the precursory oxide or the calcined oxide. However, it ispreferred to use it in the form of the calcined oxide from theview-points of the strength and catalytic activities of the catalyst.Further, for the strength of the catalyst finally obtained, the firstcomponent should preferably be in the form of fine particles having anaverage particle size of at most 10 μm, especially at most 5 μm, asmeasured by a Coulter counter method. Accordingly, it is preferablypulverized at the stage of the precursory oxide or at the stage of thecalcined oxide. According to the above-mentioned hydrothermal treatment,the fine precursor oxide can be formed. Accordingly, when the slurryobtained by the hydrothermal treatment and containing the fineprecursory oxide, is spray-dried, small agglomerates of a precursoryoxide having the above-mentioned size can directly be obtained. For thepulverization, a conventional dry-type or wet-type pulverizer such as ahammer mill, a jet mill, a colloid mill or a sand grinder may be used.In the case of the wet-type pulverization, the first component may bemixed with the second component and/or the third component prior to thepulverization operation.

The aqueous solution containing vanadium and phosphorus as the secondcomponent of the present invention usually contains substantiallytetravalent vanadium and pentavalent phosphorus and it is preferred thatat least a part thereof is present in the form of vanadyl phosphate.

This second compartment serves as a binder for the composite oxide ofthe first component and the silica sol of the third component as acarrier, and thus it contributes to an improvement of the flowabilityand the strength of the fluidized bed catalyst. The method for preparingsuch an aqueous solution is not critical. Some examples for thepreparation will be given below.

It is usually prepared by adding and dissolving a reducing agent andvanadium pentoxide in an aqueous solution containing a phosphoric acid.The atomic ratio of phosphorus to vanadium in the aqueous solution ispreferably within a range of from 0.5 to 10. In general, the aqueoussolution containing vanadyl phosphate is unstable and it is oftendifficult to maintain the solution stably for a long period of time. Inorder to stabilize the aqueous solution, oxalic acid may be added. Theamount of the addition is such that the molar ratio of oxalic acid tothe vanadium element is at most 1.2, preferably within a range of from0.2 to 1. If the amount of oxalic acid is excessive, it adverselyaffects the mechanical strength, the bulk density and the catalyticactivities of the catalyst. In other words, the range of the molar ratioof oxalic acid to vanadium element being at most 1.2 corresponds to therange within which no vanadyl oxalate is formed. Specific examples forthe preparation of the aqueous solution are as follows.

In the first method, vanadium pentoxide is added to an aqueous solutioncontaining phosphoric acid and oxalic acid in such an amount that themolar ratio of oxalic acid to vanadium element is at most 1.7 andpreferably at least 0.7, whereby an aqueous solution containing vanadylphosphate and oxalic acid is obtained. More specifically, oxalic acid isdissolved in an aqueous acidic medium containing phosphoric acid, andvanadium pentoxide is added while the temperature is maintained byslight heating at a level where the reduction proceeds. According tothis method, after the completion of the reduction, oxalic acid will bepresent in an amount of at most 1.2 mol relative to the vanadiumelement.

In the second method, a reducing agent other than oxalic acid,preferably at least one of the reducing agents selected from inorganicreducing agents such as hydrazine hydrate or hydrochlorides andphosphates of hydrazine or hydroxylamine and organic reducing agentssuch as lactic acid, is added to an aqueous acidic solution containingphosphoric acid, and then vanadium pentoxide is added, whereby thereduction is conducted to obtain a uniform aqueous solution containingvanadyl phosphate. Thereafter, oxalic acid is preferably added.

In the third method, vanadium pentoxide, phosphoric acid and phosphorousacid were mixed in an aqueous medium, whereby tetravalent vanadium ionsare formed by the reducing agent of the phosphorous acid. When theaqueous solution containing vanadyl phosphate obtained by this method,is left to stand, crystalline solid substance showing the characteristicX-ray diffraction peaks as identified in the following Table C isprecipitated.

                  TABLE C                                                         ______________________________________                                        (Anticathode Cu--K.sub.α)                                               2θ (±0.2°)                                                              Intensity   2θ (±0.2°)                                                              Intensity                                     ______________________________________                                        11.0°                                                                            100         26.2°                                                                            10                                            12.1°                                                                            20          28.2°                                                                            20                                            14.4°                                                                            10          29.2°                                                                            15                                            15.4°                                                                            10          29.5°                                                                            10                                            17.1°                                                                            10          31.0°                                                                            25                                            21.6°                                                                            15          37.5°                                                                            10                                            22.3°                                                                            40          48.5°                                                                            10                                            22.7°                                                                            10          49.7°                                                                            15                                            ______________________________________                                    

The precipitation of such crystalline solid substance is not desirablefor the purpose of the present invention. If it is necessary to maintainthe aqueous solution in a stabilized condition for a long period oftime, it is preferred to add oxalic acid to the solution.

To the above-mentioned aqueous solution containing vanadium andphosphorus, an organic solvent such as an alcohol, a ketone or an ethermay be added, as the case requires.

According to the present invention, the above-mentioned first and secondcomponents are mixed with silica sol as the third component to obtain aslurry, which is then spray-dried to obtain solid particles as thepreliminary catalytic composition. The silica sol is preliminarilyprepared in the form of a solution containing from 10 to 50% by weightof solid, and the solution is mixed with the first and second componentsto obtain a uniform slurry. The ratio by dry weight of the first, secondand third components is preferably selected within the following range:

first component:second component:third component=1:0.1-7:0.05-4.

The dry weight of the second component is calculated on the basis of V₂O₄ for vanadium and P₂ O₅ for phosphorus.

If the amounts of the first and second components are too small relativeto the third component, the catalytic activities tend to be low,although the strength of the catalyst will thereby be improved. Further,if the amount of the second component relative to the first component isless than the above range, the strength of the catalyst tends to be low.

When the first, second and third components are mixed, it is preferredto use a wet-type mixer such as a ball mill, a rod mill, a stirringmill, a sand grinder, a ultra-homomixer, a disperser or a ultrasonicmill, to make the slurry as uniform as possible and to pulverize thesolid particles as fine as possible.

Further, when the three components are mixed, an activity promotingcomponent may be added. As the activity promoting component, there maybe mentioned compounds of iron, chromium, aluminum, titanium, cobalt,manganese or nickel, and compounds of an alkaline earth metal such ascalcium or magnesium. As such compounds, there may be mentionedinorganic salt such as oxides, hydroxides, chlorides, sulfates, nitratesand carbonates, and organic salts such as acetates or oxalates. Theamount of the addition is optionally selected within a range of from0.0002 to 0.2 mol as the metal atoms per mol of the vanadium atoms.

The slurry thus obtained is then spray-dried to obtain solid particlesas the precursory catalytic composition. The above slurry is preferablyadjusted so that the oxide concentration in the slurry is usually from10 to 40%, preferably from 15 to 30% prior to the spray-drying. If theoxide concentration in the slurry is too high, not only thetransportation of the slurry becomes difficult but also the solidparticles thereby obtained tend to have poor sphericity, whereby thecatalytic composition particles finally obtained will have poorflowability. As a spray-drying condition, the temperature of the gas inthe drying region is set within a range of from 100° to 350° C.,preferably from 100° to 200° C., more preferably from 110° to 150° C. byproperly controlling the rates of the air flow and the supply of theslurry. The temperature of the drying gas at the inlet is usually from200° to 350° C. If the temperature for the spray-drying is too high, thestrength of the catalyst composition particles tends to be low. Further,the supply of the solution and the rotational speed of the disk arepreferably controlled so that the average particle size of the solidparticles after the spray-drying becomes to be within a range of from 30to 100 μm, more preferably from 40 to 70 μm.

The solid particles thereby obtained are further calcined to obtain anoxidation catalyst composition. The calcination is conducted usually ata temperature of from 400° to 700° C., preferably from 450° to 600° C.As the atmosphere for the calcination, air or air containing an organicsubstance such as butane or butenes may be used. The calcination may becarried out also in an atmosphere of an inert gas such as argon ornitrogen. When a precursory oxide is used as the first component for thepreparation of the solid particles, the precursory oxide in the solidparticles is transformed into a calcined oxide by this calcination.

The catalytic composition obtained in the foregoing manner has superiorflowability, strength and catalytic activities and thus suitable for useas a catalyst for a fluidized bed reaction. Further, the catalystcomposition of the present invention has a catalytic activity equal toor superior to the single use of the calcined oxide. For instance, thesolid particles obtained by the spray-drying may be molded by aconventional manner to obtain a catalyst useful for a fixed bedreaction.

Among the catalyst compositions obtainable by the process of the presentinvention, the catalytic compositions particularly suitable for use inthe fluidized bed reaction have the following specific physicalproperties. Namely, the catalyst compositions have features such that(i) the content of the crystalline composite oxide is from 15 to 80% byweight, (ii) the atomic ratio of phosphorus to vanadium is from 0.8 to1.5, (iii) the pore volume of the pores having a pore radius within arange of from 37 to 2000 Å is from 0.03 to 0.3 ml/g, (iv) the porevolume of the pores having a pore radius within a range of from 100 to350 Å is at least 50% of the pore volume of the pores having a poreradius within a range of from 37 to 2000 Å, (v) the specific surfacearea is from 0.5 to 20 m² /g, (vi) the average particle size is from 30to 100 μm and (vii) the shape of the catalyst particles is substantiallyspherical.

The catalyst compositions having such features are prepared by mixingthe above-mentioned first, second and third components in apredetermined ratio to obtain a slurry as uniform as possible andspray-drying the slurry, followed by calcination. The ratio of thefirst, second and third components must be selected to bring the contentof the crystalline complex oxide, the atomic ratio of phosphorus tovanadium and the pore size distribution of the catalyst compositionthereby obtained, within the above-mentioned respective ranges. Theratio by dry weight of the first, second and third components isselected usually within the range:

first component:second component:third component=1:0.1-7:0.05-4,preferably 1:0.3-4:0.5-2. The amount of the first component is selectedso that the content of the crystalline composite oxide in the catalystcomposition thereby obtained is usually within a range of from 15 to 80%by weight, preferably from 20 to 55% by weight. The first component i.e.the calcined oxide is an active component, and if the amount of thefirst component is less than 15% by weight, the catalytic activity tendsto be poor. On the other hand, if the amount exceeds 80% by weight, thestrength of the catalyst tends to be poor. The amount of the secondcomponent is selected so that the atomic ratio of phosphorus to vanadium(hereinafter referred to as "a P/V atomic ratio") in the entire catalystcomposition is within a range of from 0.8 to 1.5, preferably from 1.1 to1.3. Namely, the P/V atomic ratio of the first component is usually 1,and the overall P/V atomic ratio will be adjusted by the amounts ofphosphorus and vanadium contained in the second component including acertain amount of phosphorous and vanadium which may accompany thecrystalline composite oxide during the preparation of the firstcomponent. If the P/V atomic ratio is less than 0.8, the selectivity ofthe catalyst tends to be poor, and if the P/V atomic ratio exceeds 1.5,the catalytic activity of the catalyst tends to be poor. As mentionedabove, the second component is an aqueous solution containing vanadiumand phosphorus. However, in the final form of the catalyst composition,it forms an amorphous vanadium-phosphorus composite oxide. Accordingly,although the second component itself has a substantially lower activitythan the first component, it is highly effective to disperse the firstcomponent into the catalyst and particularly it serves to effectivelymask fine pores having a pore size of less than 100 Å which areparticularly undesirable for a selective oxidation catalyst, whereby itserves to improve the overall catalytic activity.

The catalyst composition obtained in the foregoing manner is porous. Thepore volume of the pores having a pore radius within a range of from 37to 2000 Å is from 0.03 to 0.3 ml/g, and at least 50% of the pore volumeis assumed by the pores having a pore radius within a range of from 100to 350 Å. The pore volume is measured by a mercury penetration methodwhich is commonly used. If the pore volume is too small, the catalyticproperty tends to be poor. On the other hand, if the pore volume is toogreat, the mechanical strength of the catalyst tends to be extremelypoor. Further, if the proportion of the pores having a pore radiuswithin a range of from 100 to 350 Å, i.e. the pores in a so-called mesopore range, is greater, the catalytic activity is improved. Forinstance, even when the first component as the active component ispresent in the catalyst composition only at a level of 35%, the catalysthas an activity as high as that of the calcined oxide itself. Further, apreferred catalyst composition has a substantially spherical shape andits average particle size is from 30 to 100 μm, preferably from 40 to 70μm and its specific surface area is from 0.5 to 20 m² /g. The specificsurface area of the catalyst composition can be controlled mainly by theamount of the second component and the calcination temeprature. If thespecific surface area is excessively small, the catalytic activity willbe poor. On the other hand, if the specific surface area is too great,selectivity tends to be poor. Accordingly it is preferred that thespecific surface area be adjusted within the above-mentioned range.

The oxidation catalyst composition obtained by the process of thepresent invention has superior flowability, strength and catalyticactivity and is suitable for use as a catalyst for the production ofmaleic anhydride by the oxidation of a hydrocarbon having at least fourcarbon atoms, particularly, n-butane.

The present invention will be described in further detail with referenceto Examples. However, it should be unerstood that the present inventionis by no means restricted to such Examples.

EXAMPLE 1 (A) Preparation of the first component (a calcined oxide)

Into a 100 l tank with glass lining and equipped with a stirrer, 40 l ofdeionized water was introduced, and 9.22 kg of phosphoric acid (85%,special grade reagent), 1.30 kg of hydrazine dihydrochloride and 550 gof hydrazine monohydrochloride were added and dissolved. The solutionwas heated to 75° C. and 7.28 kg of vanadium pentoxide was graduallyadded under stirring. After the addition of the total amount, thesolution was boiled for one hour to complete the reduction. The solutionwas concentrated by a rotary evaporator under reduced pressure until theamount of the solution became about 1/2. The concentrated solution wasput in six evaporation vessels and evaporated at 170° C. to dryness.After confirming that the constant weight was reached, the dried productwas roughly pulverized and the solid was boiled and washed with water,followed by filtration to completely remove the remaining hydrochloricacid. After washing with water, the solid was again dried at atemperature of 170° C. and finely pulverized by a hammer mill to obtaina precursory oxide having a P/V atomic ratio of 1.

A portion of this precursory oxide was calcined at 500° C. for 2 hoursin a nitrogen stream and then calcined at the same temperature for 1hour under an air stream. The first component thereby obtained wasconfirmed by the X-ray diffraction measurement to show the diffractionpeaks as identified in Table B.

(B) Preparation of the second component

Into 1000 ml of deionized water, 1865 g of phosphoric acid (85%, specialgrade reagent) and 1500 g of oxalic acid were dissolved, and thesolution was heated to 80° C. Then, 1082 g of vanadium pentoxide wasgradually added and dissolved therein. The P/V atomic ratio in thesolution was 1.36 and the solution was a slightly viscous blue coloreduniform solution. The solution was concentrated to some extent to bringthe weight of the solution to 4.75 kg and to adjust the oxideconcentration to 45.0% by weight as calculated as (V₂ O₄ +P₂ O₅).

(C) Preparation of a catalyst composition

The first component obtained in Example 1 (A), the second componentobtained in Example 1 (B) and a silica sol slurry (SiO₂ concentration:20% by weight) as the third component were mixed to obtain a slurry tobe spray-dried. The mixing ratio was as follows:

first component 1.467 kg

second component 1.087 kg

third component 2.446 kg

The mixture was thoroughly mixed for 40 minutes by a homogenizer toobtain a uniform slurry to be spray-dried. The slurry was sprayed by ahigh speed rotary disk rotating at a speed of 15,000 rpm and contactedand dried with a high temperature drying air (inlet temperature: 278°C.). The supply rate of the slurry was 16 l/hr. The solid particlesthereby obtained was calcined at 500° C. for 2 hours in a nitrogenstream to obtain a catalyst composition. The solid particles were sievedto obtain particles having a particle size within a range of from 25 to88 μm and the particles thereby obtained were subjected to the activitytest and the strength test. The strength was determined in such a mannerthat under the fluidized condition, the catalyst composition particleswere driven at a high speed to collide against a metal plate and thedestruction loss (%) within 2 hours was measured and taken as an indexof the mechanical strength. The greater the strength of the catalyst,the smaller the value of the destruction loss.

EXAMPLE 2 (C) Preparation of a catalyst composition

A catalyst composition was obtained by conducting the spray-drying, thecalcination and the sieving in the same manner as in Example 1 exceptthat instead of the first component (the calcined oxide) in Example 1(C), 1.639 kg of the precursory oxide having a P/V atomic ratio of 1obtained in Example 1 (A) was used without being calcined.

EXAMPLES 1 AND 2 (D) Reaction Example

With use of an air-gas mixture containing 4 molar % of n-butane and 20ml of the catalyst, an activity test was conducted at GHSV 500 by meansof a small fluidized bed reactor. The results thereby obtained are shownin Table 1. It is evident that when the precursory oxide waspreliminarily calcined, the strength and catalytic activity of thecatalyst were improved.

                  TABLE 1                                                         ______________________________________                                                         Optimum    Conversion                                                                            Yield of                                          Strength temperature                                                                              of n-butane                                                                           maleic an-                                Catalyst                                                                              (%)      (°C.)                                                                             (%)     hydride (%)                               ______________________________________                                        Example 1                                                                             10       440        78.0    37.5                                      Example 2                                                                             21       440        78.0    34.1                                      ______________________________________                                    

EXAMPLE 3 (A) Preparation of the first component (a precursory oxide;A-1)

Into 40 l of deionized water, 9.22 kg of phosphoric acid (85%, specialgrade reagent), 1.30 kg of hydrazine dihydrochloride and 550 g ofhydrazine monohydrochloride were dissolved, and the solution was heated.After the temperature reached 75° C., 7.28 kg of vanadium pentoxide wasgradually added under stirring, and after the addition of the totalamount, the solution was boiled for 1 hour to complete the reduction.This solution was concentrated by a rotary evaporator under reducedpressure until the amount of the solution became about 1/2. Theconcentrated solution was placed in an evaporation vessel and evaporatedat 170° C. to dryness. After confirming that the constant weight wasreached, the dried product was roughly pulverized and the solid therebyobtained was boiled and washed with water to completely remove theremaining hydrochloric acid. After filtration and washing with water,the solid was dried again at 170° C. in a dryer and then finelypulverized in a hammer mill to obtain the first component (A-1) having aP/V atomic ratio of 1. The average particle size was 6.5 μm. The X-raydiffraction spectrum of this first component was the same as shown inTable A.

(B) Preparation of the second component (B-1)

In 500 ml of deionized water, 1635 g of phosphoric acid (85% solution)and 1500 g of oxalic acid were dissolved, and the solution was heated to80° C. Then, 1082 g of vanadium pentoxide was gradually added anddissolved therein. The P/V atomic ratio of the solution was 1.2, and thesolution was a slightly viscous blue-colored uniform solution. Thesolution was concentrated to some extent to bring the oxideconcentration to 45.0% by weight as calculated as (V₂ O₄ +P₂ O₅) and toobtain the second component (B-1). The molar ratio of the remainingoxalic acid to the vanadium element was 0.5 since 1 mol of oxalic acidper mol of vanadium pentoxide was used in the reduction reaction.

(C) Preparation of a catalyst composition (Catalyst No. 1)

The first component obtained in Example 3 (A), the second componentobtained in Example 3 (B) and silica sol as the third component weremixed in the ratio as shown in Table 3. They were thoroughly mixed by ahomogenizer and then the mixture was spray-dried to obtain solidparticles. The solid particles thereby obtained were calcined at 500° C.for 2 hours in a nitrogen stream to obtain a catalyst composition(Catalyst No. 1). The catalyst composition was sieved to obtainparticles having a particle size within a range of from 25 to 88 μm andthe particles were subjected to the activity test and the strength test.The strength of the catalyst was determined in such a manner that in afluidized condition, the catalyst particles were driven to collideagainst a metal plate and the destruction loss (%) within 2 hours wasmeasured and taken as an index of the mechanical strength. The greaterthe strength of the catalyst, the smaller the value of the destructionloss. The results thereby obtained are shown in Table 3.

EXAMPLE 4 (B) Preparation of the second component (B-2)

A blue-colored uniform solution B-2 having a calculated oxideconcentration of 45.0% by weight was obtained in the same manner as inExample 3 (B) except that in Example 3 (B), the amount of oxalic acidwas changed to 1125 g. The P/V atomic ratio of this solution was 1.2.The molar ratio of the remaining oxalic acid to the vanadium element was0.25.

(C) Preparation of a catalyst composition (Catalyst No. 2)

A catalyst composition (Catalyst No. 2) was prepared in the same manneras in Example 3 (C) except that in Example 3 (C), the second component(B-2) obtained in Example 4 (B) was used instead of the second component(B-1) of Example 3 (B). The results thereby obtained are shown in Table3.

EXAMPLE 5 (A) Preparation of the first component (precursory oxide A-2)

In 1.9 l of deionized water, 691.8 g of phosphoric acid, 104.7 g ofhydrazine dihydrochloride and 30.0 g of hydrazine monohydrochloride weredissolved and heated. After the temperature reached 75° C., 518.4 g ofvanadium pentoxide was gradually added under stirring, and after theaddition of the total amount, the mixture was boiled for 1 hour tocomplete the reduction. After cooling, a solution prepared by dissolving81.0 g of ferric chloride (FeCl₃.6H₂ O) in 100 g of water, was addedthereto. This solution was concentrated until the amount of the solutionbecame about 2/3 and then evaporated at a temperature of 170° C. in aevaporating vessel to dryness. After confirming that the constant weightwas reached, the dried product was treated in the same manner as inExample 1 to obtain the first component (A-2) having a P/V/Fe atomicratio of 1/0.9./0.1. The X-ray diffraction peaks of this product werewithin the range of ±0.2° of the positions of the respective peaks shownin Table A.

(C) Preparation of a catalyst composition (Catalyst No. 3)

A catalyst composition (Catalyst No. 3) was prepared in the same manneras in Example 3 (C) except that in Example 3 (C), the first component(A-2) obtained in Example 5 (A) was used instead of the first component(A-1) of Example 3 (A). The results thereby obtained are shown in Table3.

EXAMPLES 6 TO 10 (A) Preparation of the first components (precursoryoxides A-3 to A-7)

The first components A-3 to A-7 having the elemental ratios are shown inthe following Table 2 were prepared in the same manner as in Example 5(A) except that instead of the ferric chloride used as the substitutioncomponent in Example 5 (A), chromium chloride, aluminum chloride,titanium oxalate, cobalt chloride and magnesium chloride wererespectively used. As the atomic ratios were modified, the amounts ofthe starting materials in Example 5 (A) were adjusted accordingly. InTable 2, A-1 and A-2 are also presented.

                  TABLE 2                                                         ______________________________________                                               Substitution                                                                             Atomic ratio                                                No.      component (Me)                                                                             V        Me     P                                       ______________________________________                                        A-1      --           1        0      1                                       A-2      Fe           0.9      0.1    1                                       A-3      Cr           0.95     0.05   1                                       A-4      Al           0.9      0.1    1                                       A-5      Ti           0.9      0.1    1                                       A-6      Ti/Co        0.94     0.04/0.02                                                                            1                                       A-7      Mg           0.95     0.05   1                                       ______________________________________                                    

(C) Preparation of catalyst compositions (Catalysts Nos. 4 to 8)

Catalyst compositions (Catalysts Nos. 4 to 8) were prepared in the samemanner as in Example 3 (C) except that in Example 3 (C), the firstcomponents (A-3 to A-7) obtained in Examples 6 (A) to 10 (A) were usedinstead of the first component (A-1) of Example 3 (A). The resultsthereby obtained are shown in Table 3.

                                      TABLE 3                                     __________________________________________________________________________    Preparation of Catalysts for a Fluidized Bed Reaction                                                     Supply                                                                             Temp. of                                                           Third rate of                                                                            gas at the                                                                          Rotational                                       First Second                                                                              Component                                                                           the solu-                                                                          inlet of the                                                                        speed of                               Example                                                                            Catalyst                                                                           Component                                                                           Component                                                                           (%)*  tion spray-drier                                                                         the disk                                                                            Strength                         No.  No.  (kg)  (kg)  (kg)**                                                                              (l/hr)                                                                             (°C.)                                                                        (rpm) (%)                              __________________________________________________________________________    3    1    A-1   B-1   20    11.0 300   15,000                                                                              13                                         1.12  1.73  2.22                                                    4    2    A-1   B-2   20    15.9 278   15,000                                                                              21                                         1.59  1.05  2.36                                                    5    3    A-2   B-1   20    10.6 300   12,000                                                                              12                                         1.12  1.73  2.22                                                    6    4    A-3   B-1   20    10.7 300   12,000                                                                              16                                         1.12  1.73  2.22                                                    7    5    A-4   B-1   20    10.6 300   12,000                                                                              10                                         1.12  1.73  2.22                                                    8    6    A-5   B-1   20    10.5 300   12,000                                                                              15                                         1.12  1.73  2.22                                                    9    7    A-6   B-1   20    10.7 300   12,000                                                                               9                                         1.12  1.73  2.22                                                    10   8    A-7   B-1   20    10.5 300   12,000                                                                              10                                         1.12  1.73  2.22                                                    __________________________________________________________________________     *Silica sol, SiO.sub.2 concentration                                          **Weight of the silica sol slurry                                        

EXAMPLES 3 TO 10 (D) Reaction Example 1

With respect to the catalyst compositions obtained in Examples 3 (C) to10 (C), 20 ml of each catalyst was used and the reaction was conductedby passing an air-gas mixture containing 4 molar % of n-butanetherethrough at a flow rate of GHSV 500. The product was absorbed inwater, and the yield was determined by potentiometric titration andanalysis of the exhaust gas composition. The results thereby obtainedare shown in Table 4.

                  TABLE 4                                                         ______________________________________                                                        Optimum              Yield of                                                 reaction   Conversion of                                                                           maleic                                   Example                                                                              Catalyst temperature                                                                              n-butane  anhydride                                No.    No.      (°C.)                                                                             (%)       (%)                                      ______________________________________                                        3      1        426        81.2      38.8                                     4      2        440        78.0      34.1                                     5      3        420        76.5      37.0                                     6      4        418        85.4      41.5                                     7      5        432        75.4      36.9                                     8      6        412        78.8      37.0                                     9      7        405        79.5      39.0                                     10     8        433        76.2      38.1                                     ______________________________________                                    

(D) Reaction Example 2

With use of the catalysts obtained in Examples 3 and 5, the influence ofthe reaction conditions was investigated.

The reaction was conducted in the same manner as in the above (D)Reaction Example 1 except that only the n-butane concentration wasvaried while maintaining the flow rate to be constant at GHSV=500, andthe performance of the reaction was investigated. The results are shownin Table 5.

                  TABLE 5                                                         ______________________________________                                                       Con-      Optimum                                                                              Conver-                                                                              Yield of                               Exam-          centration                                                                              reaction                                                                             sion of                                                                              maleic                                 ple   Catalyst of n-butane                                                                             temp.  n-butane                                                                             anhydride                              No.   No.      (mol ratio)                                                                             (°C.)                                                                         (%)    (%)                                    ______________________________________                                        3     1        4.1       430    77.0   36.5                                                  3.0       420    82.0   45.6                                                  1.6       410    86.2   47.2                                   5     3        4.0       420    76.5   37.0                                                  2.9       410    80.0   43.5                                                  1.6       390    84.5   49.9                                   ______________________________________                                    

EXAMPLE 11 (B) Preparation of the second component (B-3)

A slurry was obtained by mixing 1.33 kg of V₂ O₅, 0.844 kg of 85%phosphoric acid, 0.660 kg of phosphorous acid (purity: 97.6%) and 4.5 lof water. The slurry was boiled and refluxed for 10 hours while stirringand supplying a small amount of nitrogen. V₂ O₅ was completely dissolvedto give a blue-colored uniform solution. The solution was diluted to 10l to obtain a solution of the second component (B-3). The calculatedoxide concentration of this solution was about 18.5% by weight. The P/Vatomic ratio of this solution was 1.05. The solution was stored in acool dark place. When in use, an optional amount of phosphoric acid wasadded to adjust the phosphoric acid content.

(C) Preparation of a catalyst composition (Catalyst No. 9)

A uniformly gelled slurry was obtained by mixing 1.12 kg of the firstcomponent obtained in Example 3 (A), 3.38 l of the aqueous solutionobtained as the second component in Example 11 (B), 2.22 kg of a 20%silica sol solution as the third component and 77 kg of 85% phosphoricacid and then stirring the mixture for 60 minutes by a homogenizer. Theoxide concentration was 30% by weight. This slurry was spray-dried toobtain solid particles. The solid particles were calcined at 500° C. for2 hours in a nitrogen stream to obtain a catalyst composition (CatalystNo. 9). The catalyst composition was sieved to obtain particles having aparticle size within a range of from 25 to 88 μm and the particles wereused for a reaction.

EXAMPLE 12 (C) Preparation of a catalyst composition (Catalyst No. 10)

A catalyst composition (Catalyst No. 10) was prepared in the same manneras in Example 11 (C) except that the amount of the phosphoric acid was130 g.

EXAMPLES 11 AND 12 (D) Reaction Example

With respect to the catalyst compositions obtained in Examples 11 and12, 20 ml of each catalyst was used and a fluidized bed reaction wasconducted by passing an air-gas mixture containing 4 molar % of n-butanetherethrough at a flow rate of GHSV 500. In the same manner as inExamples 3 to 10 (D) Reaction Example 1, the product was absorbed inwater and the yield was determined by potentiometric titration andanalysis of the exhaust gas composition. The results thereby obtainedare shown in Table 6.

                  TABLE 6                                                         ______________________________________                                                        Optimum              Yield of                                                 reaction   Conversion of                                                                           maleic                                   Example                                                                              Catalyst temperature                                                                              n-butane  anhydride                                No.    No.      (°C.)                                                                             (%)       (%)                                      ______________________________________                                        11      9       463        85.7      39.4                                     12     10       476        84.3      40.2                                     ______________________________________                                    

EXAMPLE 13 (A) Preparation of the first component (Precursory oxide)

In a 100 l container with glass lining and provided with a jacket, 24.6l of water and 14.156 kg of 85% phosphoric acid were mixed and 1.73 kgof a 85% hydrazine hydrate solution was added and stirred. Then, 10.635kg of vanadium pentoxide was added while paying a careful attention tothe generation of bubbles, whereby a uniform blue solution was obtained.Thereafter, the temperature was raised and after confirming thetermination of the generation of bubbles, the temperature of thesolution was raised to 120° C. The time required for the temperaturerise was about 1 hour. The heating was continued for further 12 hours atthe same temperature to complete hydrothermal treatment. A small amountof the slurry was filtered and the light blue precipitates weresubjected to an X-ray diffraction measurement, whereby it was confirmedthat the precipitates gave the X-ray diffraction peaks as shown in TableA. From the results of the elemental analysis, the composition of thesolid was found to be generally represented by the formula (V₂ O₄ ) (P₂O₅) (2H₂ O). The slurry concentration as represented by this formulacorresponds to 40% by weight. The aqueous slurry thereby obtained wasfiltered by a centrifugal filtration apparatus to obtain about 27 kg ofthe filtrate and 23.1 kg of a wet cake. The cake was dried in a hot airdrier at 170° C. until the constant weight was reached, whereby thefirst component was obtained.

(B) Preparation of the second component

The filtrate obtained by the filtration of the above-mentioned aqueousslurry was light blue thus indicating the dissolution of tetravalentvanadyl ions. To this filtrate, 9.25 kg of 85% phosphoric acid and 8.95kg of oxalic acid were added and dissolved under heating, and then 6.46kg of vanadium pentoxide was gradually added and dissolved, whereby thesecond component was obtained.

(C) Preparation of a catalyst composition

By means of a homogenizer, 5.0 kg of the dried cake obtained as thefirst component in the above (A), 13.7 kg of the second componentobtained in the above (B) and 5.0 kg of 40% silica sol solution as thethird component were thoroughly mixed for 40 minutes. The slurry therebyobtained was highly viscous and at least partially gelled. The totalconcentration of the oxides (V₂ O₄ +P₂ O₅) in the solution portion ofthe slurry and the crystalline oxide was about 40% by weight. The ratioof the crystalline oxide, the oxide in the solution portion and silicasol in the slurry was 45/35/20. The slurry thereby obtained was spraydried by means of a spray drier. The slurry was sprayed by a rotary diskrotating at a rotational speed of 15,000 rpm and contacted and driedwith heated air. The temperature of the drying air was 270° C. at theinlet and the gas temperature at the outlet of the spray drier was 138°C.

The solid particles thereby obtained (P/V atomic ratio: 1.088) wascalcined in a fluidized condition at 500° C. for 2 hours in a nitrogenstream and thereby activated to obtain a catalyst composition. Thecatalyst composition was classified and used for a reaction.

(D) Reaction Example 1

The catalyst composition obtained in the above (C) was classified bysieving to obtain particles having a particle size within a range offrom 25 to 88 μm. The average particle size of the classified particleswas 56 μm. 50 ml of the particles were filled in a small fluidized bedreactor. Air-gas mixtures having various concentrations of n-butane wereintroduced into the reactor and reacted at a flow rate of GHSV 500. Theproducts thereby obtained were respectively absorbed in water andquantitatively analyzed by potentiometric titration and gaschromatography analysis of the exhaust gas. The results of the reactionsare shown in Table 7.

                  TABLE 7                                                         ______________________________________                                        Concentration of                                                                         Optimum    Conversion Yield of maleic                              n-butane   temperature                                                                              of n-butane                                                                              anhydride                                    (molar %)  (°C.)                                                                             (%)        (%)                                          ______________________________________                                        4.05       440        77.0       37.5                                         2.95       420        82.0       45.6                                         1.58       410        86.2       47.2                                         ______________________________________                                    

EXAMPLE 14 (A) Preparation of the first component (Precursory oxide) andthe second component

In a 10 l beaker, 622.55 g of 85% phosphoric acid, 73.6 g of a 85%hydrazine hydrate aqueous solution and 2550 g of deionized water weremixed, and then 454.75 g of vanadium pentoxide was added and stirred.With the generation of bubbles, the temperature of the solution rised toabout 60° C. When the generation of bubbles was substantiallyterminated, the solution was heated to the boiling point to complete thereaction.

This solution was boiled and concentrated until the weight of thesolution became 2500 g. Then, the concentrated solution was transferredto a 2.5 l autoclave and heated to 130° C., and the hydrothermaltreatment was conducted for 5 hours. The resulting viscous slurry wascooled and transferred with a small amount of water to a 10 l beaker.Then, 384.46 g of 85% phosphoric acid, 382.80 g of oxalic acid, 600 mlof water and 276.16 g of vanadium pentoxide were added, and thetemperature was slowly raised under stirring. With the generation ofbubbles, the reduction proceeded and after the entire mixture turnedinto a blue-colored slurry, the slurry was further boiled to completethe reduction.

The slurry was concentrated until the weight of the entire slurry became3200 g. The total concentration of the oxides (V₂ O₄ +P₂ O₅) in thesolution portion of the slurry and the crystalline oxide in the slurrythereby obtained was 40% by weight. The weight ratio of the crystallineoxide and the oxides in the solution portion was about 58:42. A smallamount of solid was filtered from the slurry and subjected to an X-raydiffraction measurement, whereby the X-ray diffraction peaks were foundto be identical with the X-ray diffraction peaks shown in Table A. Theseare the same as the X-ray diffraction peaks of the crystals in theslurry obtained by the hydrothermal treatment, whereby it was found thatno change in the crystal structure took place during the secondarydissolving step of the vanadium pentoxide.

(C) Preparation of a catalyst composition

To the aqueous slurry of the first and second components obtained in theabove (A), 1800 g of 30% silica sol was added and the mixture wasthoroughly mixed by a homogenizer to obtain a viscous slurry to be spraydried. The slurry was spray-dried under the same conditions as inExample 13 (C). The solid particles thereby obtained was calcined at350° C. for 1 hour in an air stream and then at 500° C. for 2 hours in anitrogen stream and thereby activated to obtain a catalyst composition.The catalyst composition was classified and used for a reaction. TheSiO₂ content in the catalyst as calculated as oxides was 30% by weight.

EXAMPLE 15 (A) Preparation of the first component (Calcined oxide)

In the same manner as in Example 13 (A), a slurry containing thecrystalline oxide was obtained and filtered by a centrifugal filtrationapparatus, and the cake thereby obtained was dried. The cake waspulverized into small aggregates and calcined for 2 hours in a nitrogenstream in a calcination tube. The X-ray diffraction peaks of thecalcined product were the same as those shown in Table B. About 18.0 kgof the small aggregates thereby obtained were finely pulverized by ahammer mill to obtain the first component.

(B) Preparation of the second component

To 3500 g of the filtrate obtained by the filtration of the aqueousslurry in Example 15 (A), 1180 g of 85% phosphoric acid and 800 g ofoxalic acid were added and dissolved under heating, and then 576.6 g ofvanadium pentoxide was gradually added and dissolved to obtain thesecond component.

(C) Preparation of a catalyst composition

To the second component obtained in Example 15 (B), 1785 g of a 40%silica sol solution was added and 1000 g of the finely pulverized firstcomponent obtained in Example 15 (A) was added. The mixture wasthoroughly mixed by a homogenizer to obtain a slurry to be spray-dried.The slurry was spray-dried under the same condition as in Example 13(C). The solid particles thereby obtained were calcined at 350° C. for 1hour in an air stream and then at 500° C. for 2 hours in a nitrogenstream and thereby activated to obtain a catalyst composition. Thecatalyst composition was classified and used for a reaction. The SiO₂content in the catalyst as calculated as oxides was 25% by weight.

EXAMPLES 14 AND 15 (D) Reaction Example

The catalysts obtained in Examples 14 (C) and 15 (C) were classified inthe same manner as in the Reaction Example 13 (D) and subjected to theactivity test under the same conditions. The results thereby obtainedare shown in Table 8.

                  TABLE 8                                                         ______________________________________                                                                             Yield of                                 Exam- Concentration                                                                             Optimum    Conversion                                                                            maleic                                   ple   of n-butene temperature                                                                              of n-butane                                                                           anhydride                                No.   (molar %)   (°C.)                                                                             (%)     (%)                                      ______________________________________                                        14    4.0         420        76.5    37.0                                     15    4.0         460        76.7    39.7                                     ______________________________________                                    

EXAMPLE 16 (A) Preparation of the first component (Calcined oxide)

Into a 100 l pressure container with glass lining and provided with ajacket, 38.0 kg of deionized water, 21.83 kg of 85% phosphoric acid and2.85 kg of a 80% hydrazine hydrate solution were fed and stirred toobtain a uniform solution. While paying a careful attention to thegeneration of bubbles, 16.40 kg of vanadium pentoxide was graduallyadded thereto and dissolved. During this operation, a cooling medium wascirculated in the jacket to maintain the temperature of the solution ata level of from 60° to 80° C. After the dissolution was completed andthe generation of bubbles ceased, 1.0 kg of seed crystals of theprecursory oxide was added and the solution was heated under a closedcondition by circulating a high temperature medium preliminarily heatedto a temperature of 160° C. in the jacket. The temperature was raised to140° C. in 1.5 hours. The heating and stirring were continued for 10hours. During the period, the inner pressure of the container wasconstant at a level of about 2.4 kg/cm² G. After cooling the slurry to90° C., 10.3 kg of deionized water was added and the content waswithdrawn and left for cooling. A small amount of this slurry wasfiltered, and the light blue solid thereby obtained was subjected to anX-ray diffraction measurement, whereby the X-ray diffraction peaks werefound to be identical with the X-ray diffraction peaks shown in Table A.After being uniformly mixed by a stirrer, the slurry was spray-dried bya spray drier having a high speed rotary disk to obtain a fine powderprecursory oxide. The spray-drying was conducted under such conditionthat the temperature of the gas was from 330° to 370° C. at the inletand 160° C. at the outlet. The P/V atomic ratio of this powder was 1.05.

The precursory oxide thus obtained was calcined in a small rotary kilnat a temperature of 520° C. for a retention time of 15 minutes in anitrogen gas stream. The X-ray diffraction spectrum of the calcinedoxide was identical with that shown in Table B. Further, the ratio ofthe pentavalent vanadium in the total vanadium in the calcined oxide was0%.

(B) Preparation of the second component

In 50 kg of deionized water, 6.929 kg of 85% phosphoric acid and 4.789kg of oxalic acid (H₂ C₂ O₄.2H₂ O) were added and dissolved understirring and heating to 80° C. While paying a careful attention to thegeneration of bubbles, 4.319 kg of vanadium pentoxide was added theretoand dissolved. The solution was cooled and water was added to bring thetotal weight to 67.1 kg, whereby the second component was obtained.

(C) Preparation of a catalyst composition

To 20.0 kg of the second component obtained in Example 16 (B), 3.82 kgof a 40% silica sol solution as the third component and 2.14 kg of thefirst component (the calcined oxide) obtained in Example 16 (A) wereadded to obtain a slurry. This slurry was subjected to a continuouswet-type mill to obtain a sufficiently uniform slurry and thenspray-dried by a spray drier provided with a high speed rotary disk. Thespray-drying was conducted under such condition that the temperature ofthe gas was from 200° to 210° C. at the inlet and from 120° to 130° C.at the outlet. The average particle size of the solid particles therebyobtained was within a range of from 58 to 62 μm. The spray-dried productwas calcined in a fluidized calcination furnace at a temperature of 350°C. for 1 hour in an air stream and further calcined at 500° C. for 2hours in a nitrogen gas stream and thereby activated to obtain acatalyst composition (Catalyst No. 11). The catalyst composition wassieved to obtain particles having a particle size within a range of from44 to 116 μm and the particle were used for a reaction.

EXAMPLE 17 (A) Preparation of the first component (Calcined oxide)

The precursory oxide obtained in Example 16 (A) was calcined in a smallrotary kiln at a temperature of 520° C. for a retention time of 15minutes in a nitrogen gas stream and then further calcined in the samerotary kiln at a temperature of 580° C. for a retention time of 15minutes under an air stream. The X-ray diffraction peaks of the calcinedproduct were identical with those shown in Table B and the ratio of thepentavalent vanadium in the total vanadium was 21.7%.

EXAMPLE 18 (A) Preparation of the first component (Calcined oxide)

The precursory oxide obtained in Example 16 (A) was calcined in a smallrotary kiln at a temperature of 500° C. for a retention time of 15minutes in a stream of air diluted with nitrogen (oxygen concentration:2%). The X-ray diffraction peaks of this product were identical withthose shown in Table B and the ratio of the pentavalent vanadium in thetotal vanadium was 16.3%.

EXAMPLE 19 (A) Preparation of the first component (Calcined oxide)

The precursory oxide obtained in Example 16 (A) was put in an amount of1 kg in each of porcelain calcination dishes having a capacity of 2 l.The dishes were piled in a spaced manner in a muffle furnace havinginner volume of 500 l. The interior of the furnace was flushed with anitrogen gas and then heated and the precursory oxide was calcined at550° C. for 2 hours. Then, air was gradually introduced into the furnaceand heated at a temperature of 550° C. for 1 hour and then cooled. TheX-ray diffraction peaks of the calcined product were identical withthose shown in Table B and the ratio of the pentavalent vanadium in thetotal vanadium was 23.4%.

EXAMPLE 20 (A) Preparation of the first component (Calcined oxide)

A calcined oxide was prepared in the same manner as in Example 19 (A)except that in Example 19 (A), the calcination temperature was changedto 600° C. The X-ray diffraction peaks of the calcined oxide therebyobtained were subtantially the same as those shown in Table B. However,a broad weak peak was observed at 2θ=21.4°. The ratio of the pentavalentvanadium in the total vanadium was 35.2%.

EXAMPLES 17 TO 20 (C) Preparation of catalyst compositions (CatalystsNos. 12 to 15)

Catalyst compositions (Catalysts Nos. 12 to 15) were obtained in thesame manner as in Example 16 (C) except that in Example 16 (C), thefirst components obtained in Examples 17 (A) to 20 (A) were respectivelyused instead of the first component of Example 16 (A).

EXAMPLES 16 TO 20 (D) Reaction Example

The catalyst compositions (Catalysts Nos. 1 to 5) obtained in Examples16 (C) to 20 (C) were respectively introduced into fluidized bedreactors made of heat resistant glass, and air containing 3% by volumeof n-butane was introduced thereinto at a flow rate of GHSV 500 to carryout the production of maleic anhydride. The products were absorbed inwater. The reaction products were analyzed by potentiometric titrationand gas chromatography of the exhaust gas. The results thereby obtainedare shown in Table 9.

                  TABLE 9                                                         ______________________________________                                                     Ratio of the                                                                              Optimum                                                                              Con-                                                       pentavalent reaction                                                                             version                                                                              Yield of                               Exam- Cat-   vanadium in temper-                                                                              of     maleic                                 ple   alyst  the first   ature  n-butane                                                                             anhydride                              No.   No.    component (%)                                                                             (°C.)                                                                         (%)    (%)                                    ______________________________________                                        16    1      0           460    70.5   34.7                                   17    2      21.7        420    94.0   50.9                                   18    3      16.3        431    89.1   49.2                                   19    4      23.4        420    83.5   50.6                                   20    5      35.2        433    90.4   46.3                                   ______________________________________                                    

EXAMPLE 21 (A) Preparation of the first component (Precursory oxide)

Into a 100 l pressure container with glass lining and provided with ajacket, 38.0 kg of deionized water, 21.83 kg of 85% phosphoric acid and2.85 kg of a 80% hydrazine hydrate solution were fed, and then whilepaying a careful attention to the generation of the bubbles, 16.40 kg ofvanadium pentoxide powder was gradually added and dissolved understirring. During this period, a cooling medium was circulated in thejacket to control the temperature rise due to the exothermal reactionand to maintain the temperature of the solution at a level of from 60°to 80° C. The addition of vanadium pentoxide was completed in about 4hours, whereby a blue-colored vanadyl phosphate solution was obtained.To this solution, 1.0 kg of seed crystals were added and the solutionwas heated by circulating a heating medium heated to 160° C. in thejacket. The temperature of the solution was raised to 140° C. in 2 hoursand hydrothermal treatment was continued for 10 hours at thetemperature. During this period, the pressure was about 2.4 kg/cm² G.After cooling the reaction mixture to 90° C., 10.3 kg of deionized waterwas added to adjust the solid concentration in the slurry to about 35%and then the slurry was withdrawn to obtain the first component. Thissolid was subjected to an X-ray diffraction measurement, whereby itshowed the characteristic X-ray diffraction peaks as identified in TableA and thus it was confirmed to be a pure precursory oxide. Further, theparticle size distribution of the solid in the slurry was investigatedby a Coulter counter method, whereby the average particle size was foundto be 0.7 μm. The P/V atomic ratio based on the feed materials of thisoxide slurry was 1.05.

(B) Preparation of the second component

In 50 kg of deionized water, 6.929 kg of 85% phosphoric acid and 4.789kg of oxalic acid (H₂ C₂ O₄.2H₂ O) were added and dissolved understirring and heating to 80° C. While paying a careful attention to thegeneration of bubbles, 4.319 kg of vanadium pentoxide was graduallyadded and dissolved and then the solution was cooled. Water was added tobring the total amount of the solution to 67.1 kg. The P/V atomic ratioof this solution was 1.266 and the solution contained 0.8 mol of oxalicacid per gram atom of vanadium. This solution was stable and noprecipitation of solid took place even when stored at room temperaturefor one month.

(C) Preparation of a catalyst composition

To 20 kg of the vanadyl phosphate solution obtained as the secondcomponent in the above (B), 6.80 kg of the precursory oxide slurryobtained as the first component in the above (A) was added understirring. Then, 3.82 kg of a 40% silica sol solution as the thirdcomponent was added under stirring. This slurry was treated by acontinuous wet-type mill to obtain a suffficiently uniform slurry. Theslurry was spray-dried by means of a spray drier provided with a highspeed rotary disk. The solid concentration of the slurry was 20% and thetemperature of the drying gas was 250° C. at the inlet and 140° C. atthe outlet. The solid particles thereby obtained had an average particlesize of 58 μm and they had satisfactory sphericity and strength.

The solid particles thus obtained were introduced into a fluidized bedand calcined at 350° C. for 1 hour in an air atmosphere and furthercalcined at 500° C. for 2 hours in a nitrogen atmosphere, whereby acatalyst composition was obtained. The catalyst composition obtained bythe calcination was subjected to an X-ray diffraction measurement,whereby it showed the diffraction peaks as identified in Table B andthus it was confirmed that the precursory oxide obtained by thehydrothermal treatment was converted to a calcined oxide of the formula(VO)₂ P₂ O₇ during the preparation of the catalyst composition. Further,the intensity of the diffraction peaks was substantially the same as thestrength expected from the amount of the precursory oxide used in thepreparation of the catalyst composition. Accordingly, it is consideredthat during the process for the preparation of the catalyst composition,no destruction of the crystalline oxide took place and no conversion ofthe vanadium-phosphorus oxide in the vanadyl phosphate solution used asa binder to a crystalline oxide took place.

(D) Reaction Example

The catalyst composition obtained in Example 21 (C) was sieved to obtainparticles having a particle size within a range of from 44 to 116 μm andthe particles were subjected to an activity test by means of a smallfluidized bed reactor. Namely, 20 ml of the catalyst composition wasintroduced into a reaction tube having an inner diameter of 17 mm and anair-gas mixture containing 3% by volume of n-butane was introduced intothe reactor at a flow rate of GHSV 500 to carry out the reaction. Theproduct was absorbed in water. The performance of the reaction wasdetermined by potentiometric titration of the aqueous solution therebyobtained and gas chromatography of the exhaust gas. As a result, it wasfound that the optimum reaction temperature was 445° C., the conversionof butane at that time was 82.0% and the yield of maleic anhydride was44.0%.

EXAMPLE 22 (A) Preparation of the first component (Precursory oxide)

Into a 100 l pressure container with glass lining and provided with ajacket, 38.0 kg of deionized water, 21.83 kg of 85% phosphoric acid and2.85 kg of a 80% hydrazine hydrate solution were fed, and while paying acareful attention to the generation of bubbles, 16.40 kg of vanadiumpentoxide powder was gradually added under stirring. During this period,a cooling medium was circulated in the jacket to control the temperaturerise due to the exothermal reaction and to maintain the temperature ofthe solution at a level of from 60° to 80° C. The addition of vanadiumpentoxide was completed in about 4 hours, whereby a blue-colored vanadylphosphate solution was obtained. To this solution, 1.0 kg of seedcrystals were added and the solution was heated by circulating a hightemperature medium heated to 160° C. in the jacket. The temperature ofthe solution was raised to 140° C. in 2 hours, and the hydrothermaltreatment was continued for 10 hours at the same temperature. Duringthis period, the pressure was about 2.4 kg/cm² G. After cooling thereaction mixture to 90° C., 10.3 kg of deionized water was added toadjust the solid concentration in the slurry to about 35% and then theslurry was withdrawn. The solid thereby obtained was subjected to anX-ray diffraction measurement, whereby it was found that the solidshowed the characteristic X-ray diffraction peaks as identified in TableA and it was confirmed that the solid was a pure precursory oxide.Further, the particle size distribution of the solid in the slurry wasinvestigated by a Coulter counter method, whereby the average particlesize was found to be 0.7 μm.

The slurry thus obtained was treated by a disperser for 30 minutes andthen spray-dried by means of a spray drier provided with a high speedrotary disk, whereby a fine powder solid (Precursory oxide) wasobtained. The spray-drying was conducted under such condition that thetemperature of the gas was 360° C. at the inlet and from 150° to 160° C.at the outlet. The particle size of the fine powder solid therebyobtained was within a range of from about 5 to about 50 μm. The strengthwas low and it was found to be suitable for the subsequent step.Further, the P/V atomic ratio of this fine powder was 1.05.

(B) Preparation of the second component

In 60 kg of deionized water, 6.929 kg of 85% phosphoric acid and 5.987kg of oxalic acid (H₂ C₂ O₄.2H₂ O) were added and dissolved understirring and heating to 80° C. While paying a careful attention to thegeneration of bubbles, 4.319 kg of vanadium pentoxide was graduallyadded and dissolved, and the solution was cooled. Water was added tobring the total amount of the solution to 82.8 kg. The P/V atomic ratioof this solution was 1.266 and the solution contained 0.5 mol of oxalicacid per gram atom of vanadium. Further, this solution was stable and noprecipitation of solid took place even when stored at room temperaturefor 1 month.

(C) Preparation of a catalyst composition

To 40 kg of the vanadyl phosphate solution prepared as the secondcomponent in Example 22 (B), 6.20 kg of a 40% silica sol solution as thethird component was added under stirring, and then 3.865 kg of the finepowder solid obtained as the first component in Example 22 (A) wasadded. This slurry was treated by a continuous wet-type mill to obtain asufficiently uniform slurry. The slurry was spray-dried by means ofspray-drier. The solid concentration of the slurry was 20%, and thetemperature of the gas was 210° C. at the inlet and 130° C. at theoutlet. The solid particles thus obtained had an average particle sizeof 61 μm and had satisfactory sphericity and strength.

The solid particles were calcined at 500° C. for 2 hours in a nitrogenstream to obtain a catalyst composition. The X-ray diffraction peaks ofthis catalyst composition were the same as the X-ray diffraction peaksshown in Table B, thus indicating that a calcined oxide [(VO)₂ P₂ O₇crystal phase] was formed. Further, the intensity of the X-raydiffraction peaks was substantially the same as the intensity expectedfrom the amount of the precursory oxide used in the preparation of thecatalyst composition. Thus, it is considered that in the process for thepreparation of the catalyst composition, the crystal phase of theprecursory oxide prepared by the hydrothermal treatment in the processfor the preparation of the above first component was entirelytransformed into the crystal phase of the calcined oxide and noconversion of the vanadyl phosphate solution used as a binder to thecalcined oxide took place.

EXAMPLE 23 (A) Preparation of the first component (Calcined oxide)

The fine powder solid obtained in Example 22 (A) was divided and placedin porcelain dishes having a capacity of 2 l and then calcined in amuffle furnace. The interior of the furnace was flushed with nitrogenprior to the temperature rise, and maintained at a temperature of 500°C. for 2 hours while supplying nitrogen. Then, air was graduallyintroduced and the temperature was maintained at 500° C. for further 1hour and then lowered whereby the first compoent (Calcined oxide) wasobtained. The X-ray diffraction peaks of the calcined oxide were thesame as the X-ray diffraction peaks shown in Table B, thus indicatingthe conversion to a (VO)₂ P₂ O₇ crystal phase.

(C) Preparation of a catalyst composition

A slurry was prepared in the same manner as in Example 22 (C) exceptthat 3.47 kg of the calcined oxide of Example 23 (A) was used. Theslurry was spray-dried and calcined to obtain a catalyst composition.The solid particles obtained by the spray-drying had an average particlesize of 59 μm and had satisfactory sphericity and strength. Further, theX-ray diffraction peaks of the catalyst composition were substantiallythe same as those of Example 22.

EXAMPLES 22 AND 23 (D) Reaction Example

Into fluidized bed reactors having an inner diameter of 17 mm, 20 ml ofthe catalyst compositions obtained in Examples 22 (C) and 23 (C)(particle size; 44 to 116 μm) were respectively introduced, and aircontaining 3% by volume of n-butane was introduced into the respectivereactors at a flow rate of GHSV 500 to carry out the reaction. Theproducts were absorbed in water, and the performance of the reactionswas determined by potentiometric titration of the respective aqueoussolutions and gas chromtography of the respective exhaust gases. Theresults thereby obtained are shown in Table 10.

                  TABLE 10                                                        ______________________________________                                        Example                                                                              Optimum reaction                                                                            Conversion Yield of maleic                               No.    temperature   of n-butane                                                                              anhydride                                     ______________________________________                                        22     440° C.                                                                              86.1%      45.4%                                         23     425° C.                                                                              86.5%      49.5%                                         ______________________________________                                    

EXAMPLE 24 (A) Preparation of the first component (Precursory oxide)

In a 100 l container with glass lining and provided with a jacket, 24.6l of deionized water and 14.165 kg of phosphoric acid were mixed, andthen 1.73 kg of a 85% hydrazine hydrate solution was added and mixed.Then, while paying a careful attention to the generation of bubbles,10.635 kg of vanadium pentoxide was gradually added to obtain a uniformblue-colored solution. Thereafter, the temperature of the heating mediumwas raised, and after confirming the termination of the generation ofbubbles, the container was closed. The solution was heated to 140° C.The time required for the temperature rise was about 1.5 hours. Theheating was continued for 10 hours at the same temperature to completethe hydrothermal treatment. A small amount of solid was filtered fromthe slurry, and the light blue precipitates were subjected to an X-raydiffraction measurement, whereby the precipitates were found to show theX-ray diffraction peaks as identified in Table A. From the elementalanalysis, the composition of the solid was found to be generallyrepresented by the formula (V₂ O₄) (P₂ O₅) (2H₂ O). This slurry wasuniformly mixed and then spray-dried to obtain light blue precursoryoxide powder to be used as the first component.

(B) Preparation of the second component

In 5.0 kg of deionized water, 3.50 kg (30.357 mol) of 85% phosphoricacid and 3.025 kg (24 mol) of oxalic acid (H₂ C₂ O₄.2H₂ O) were addedand dissolved under stirring and heating to 80° C. To this solution,2.182 kg (12.0 mol) of vanadium pentoxide was gradually added anddissolved while paying a careful attention to the generation of bubbles.After cooling the reaction mixture, water was added to bring the totalamount of the solution to 11.85 kg. The P/V atomic ratio in thissolution was 1.265, and the concentration of oxides (V₂ O₄ +P₂ O₅)was35% by weight.

(C) Preparation of a catalyst composition (Catalyst No. 21)

0.379 kg of the precursory oxide powder obtained as the first componentin Example 24 (A), 1.114 kg of a solution of the second componentobtained in Example 24 (B), a solution obtained by diluting 0.675 kg ofa 40% colloidal silica sol solution as the third component with 2.911 kgof deionized water and 10 g of calcium hydroxide were mixed, and themixture was thoroughly admixed by a continuous wet-type pulverizer.Then, the mixture was spray-dried to obtain spherical solid particleshaving an average particle size of about 60 μm. By means of a quartztube calcination furnace, 60 g of the solid particles were calcined in afluidized condition in a nitrogen stream to obtain a catalystcomposition (Catalyst No. 21). The calcination was conducted at atemperature of 600° C. for 3 hours. The P/V atomic ratio in the catalystcomposition was 1.16.

EXAMPLES 25 TO 29 (A) Preparation of the first components (Calcinedoxides)

10 kg of the precursory oxide powder obtained in Example 24 (A) wasdivided and placed in porcelain dishes having a capacity of 2 l andcalcined in a muffle furnace having a capacity of 500 l. The interior ofthe furnance was purged with nitrogen prior to the temperature rise. Thetemperature was raised while supplying nitrogen, and the calcination wascarried out at 550° C. for 2 hours. Then, air was gradually introducedand the calcination was continued for further 1 hour. The powder therebyobtained was yellowish brown, and the X-ray diffraction peaks of thepowder were found to be identical with the X-ray diffraction peaks shownin Table B. The valence of vanadium was measured by anoxidation-reduction titration method, whereby V⁵⁺ /ΣV (the ratio ofpentavalent vanadium in the total vanadium) was found to be 23.4%.

(C) (Preparation of catalyst compositions (Catalysts Nos. 22 to 26)

0.340 kg of the first component obtained in the above (A), 1.114 kg ofthe solution of the second component obtained in Example 24 (B), asolution obtained by diluting 0.675 kg of a 40% colloidal silicasolution as the third component with 2.911 kg of deionized water and 10g of an alkaline earth metal compound shown in Table 10 and used as anactivity promoting component, were mixed, and the mixture was thoroughlyadmixed by a continuous wet-type mill. The mixture was then spray-driedand calcined under the same conditions as in Example 24 (C) to obtain acatalyst composition (Catalyst Nos. 22 to 26). The P/V atomic ratio inthe catalyst was 1.16, and the atomic ratio of the alkaline earth metalto vanadium was within a range of from 0.039 to 0.013.

EXAMPLE 30 (C) Preparation of a catalyst composition (Catalyst No. 27)

A catalyst composition (Catalyst No. 27) was prepared in the same manneras in Example 25 (C) except that the addition of 10 g of the alkalineearth metal compound was omitted.

EXAMPLES 24 TO 30 (D) Reaction Example

5 g of each fluidized bed catalyst coomposition obtained in Examples 24(C) to 30 (C) was pulverized in a porcelain mortar and pelletized intopellets of 5 mm φ×3 mm H by a pelletizer. The pellet were crushed toobtain particles having a particle size within a range of from 14 to 24mesh(JIS). Into a micro reactor made of hard glass and having a diameterof 6 mm, 1 ml of the catalyst was filled, and an air-gas mixturecontaining 1.5% by volume of n-butane was passed therethrough at a flowrate of GHSV 2000 to carry out the reaction. The product was passedthrough a gas sampler and directly quantitatively analysed by gaschromatography. The results of the reactions thereby obtained are shownin Table 11.

                  TABLE 11                                                        ______________________________________                                                                Reaction       Yield of                               Exam- Cat-              tempera-                                                                             Conversion                                                                            maleic                                 ple   alyst             ture   of n-butane                                                                           anhydride                              No.   No.    Additive   (°C.)                                                                         (%)     (%)                                    ______________________________________                                        24    21     Ca(OH).sub.2                                                                             468    90.9    49.7                                   25    22     Mg(OH).sub.2                                                                             453    95.1    57.9                                   26    23     Ca(OH).sub.2                                                                             452    92.8    55.2                                   27    24     Sr(OH).sub.2                                                                             455    92.0    53.0                                   28    25     Ba(OH).sub.2                                                                             463    91.9    52.8                                   29    26     CaSiO.sub.3.xH.sub.2 O                                                                   462    89.4    54.0                                   30    27     --         480    93.2    49.9                                   ______________________________________                                    

EXAMPLE 31 (A) Preparation of the first component (Precursory oxide)

Into a 100 l pressure container with glass lining and provided with ajacket, 38.0 kg of deionized water, 21.83 kg of 85% phosphoric acid and2.85 kg of a 80% hydrazine hydrate solution were fed. Then, while payinga careful attention to the generation of bubbles, 16.40 kg of vanadiumpentoxide powder was gradually added and dissolved under stirring.During this period, a cooling medium was circulated in the jacket tocontrol the temperature rise due to the exothermal reaction and tomaintain the temperature of the solution at a level of from 60° to 80°C. The addition of vanadium pentoxide was completed in about 4 hours,whereby a blue-colored vanadyl phosphate solution was obtained. To thissolution, 1.0 kg of seed crystals were added, and then the solution washeated by circulating a hot medium heated to 160° C. in the jacket. Thetemperature of the solution was raised to 140° C. in 2 hours, thehydrothermal treatment was continued for 10 hours at the sametemperature. During this period, the pressure was about 2.4 kg/cm² G.After cooling the reaction mixture to 90° C., 10.3 kg of deionized waterwas added to adjust the solid content in the slurry to about 35% andthen the slurry was withdrawn. The solid thereby obtained was subjectedto an X-ray diffraction measurement, whereby the solid was found to showthe characteristic X-ray diffraction peaks as identified in Table A andthus it was confirmed to be a pure precursory oxide. Further, theparticle size distribution of the solid in the slurry was investigatedby a Coulter counter method, whereby the average particle size was foundto be 0.7 μm. This oxide slurry was spray-dried by means of aspray-drier to obtain 29.8 kg of a light blue oxide powder. The P/Vatomic ratio based on the feed materials of the oxide slurry was 1.05.It was confirmed that the precursory oxide obtained by filtering thesample withdrawn for the measurement of the abovementioned X-raydiffraction measurement and washing it, can generally be represented bythe formula (V₂ O₄) (P₂ O₅) (2H₂ O).

(B) Preparation of the second component

As an example of a starting material suitable for an amorphous complexoxide containing phosphorus and vanadium as the major constituents, avanadyl phosphate solution was prepared. In 3.0 kg of deionized water,2.956 kg of 85% phosphoric acid was dissolved, and 2.55 kg of oxalicacid (H₂ C₂ O₄.2H₂ O) was further added and dissolved under heating. Thesolution was heated to 80° C., and 1.842 kg of vanadium pentoxide wasgradually added and dissolved while paying a careful attention to thegeneration of bubbles. Then, the heating was continued for further 10minutes under the boiling condition to complete the reduction. Thesolution was cooled, and deionized water was added to adjust the totalamount to 10.00 kg, whereby the second component was obtained.

(C) Preparation of a catalyst composition (Catalyst No. 31)

393.6 g of the dried powder of the precursory oxide obtained in Example31 (A), 1.143 kg of the vanadyl phosphate solution obtained in Example31 (B) and a solution obtained by diluting 625.1 g of a commerciallyavailable 40% colloidal silica solution with 2.84 kg of deionized water,were mixed, and then the mixture was treated by a continuous wet-typepulverizer to obtain a sufficiently uniform slurry. This slurry wasspray-dried by means of a spray-drier to obtain spherical solidparticles having an average particle size of 58 μm. The solid particleswere calcined at a temperature of 350° C. for 1 hour in an air streamand then at 500° C. for 2 hours in a nitrogen stream, whereby a catalystcomposition (Catalyst No. 31) was obtained.

EXAMPLE 32 (A) Preparation of the first component (Calcined oxide)

The precursory oxide obtained in Example 31 (A) was calcined to obtain acalcined oxide. Namely, 10 kg of the precursory oxide obtained inExample 31 (A) was divided and placed in 10 procelain dishes having acapacity of 2 l, and the porcelain dishes were placed in a mufflefurnace having a capacity of 500 l. After flushing the interior of thefurnace with nitrogen gas, the temperature was raised and thecalcination was carried out at a temperature of 550° C. for 2 hours.Then, air was gradually introduced into the furnace and the heating wascontinued for further 1 hour. Then, the furnace was cooled. As a resultof the X-ray diffraction measurement, the powder obtained by thecalcination was found to show no peaks other than the X-ray diffractionpeaks shown in Table B, and thus the powder was confirmed to be a highlypure calcined oxide. Further, the ratio of the pentavalent vanadium inthe total vanadium atoms was measured by an oxidation-reductiontitration method, whereby it was found to be 23.4%. Namely, it was foundthat at least part of vanadium in (VO)₂ P₂ O₇ absorbed oxygen andassumed the pentavalent state while maintaining the crystallinestructure.

(C) Preparation of a catalyst composition (Catalyst No. 32)

A catalyst composition (Catalyst No. 32) was prepared in the same manneras in Example 31 (C) except that 353.2 g of the calcined oxide powderobtained in Example 32 (A) was used instead of the precursory oxidepowder of Example 31 (A).

COMPARATIVE EXAMPLE 1 (C) Preparation of a catalyst composition(Catalyst No. 33)

Without using the calcined oxide powder obtained in Example 32 (A), 1.43kg of the vanadyl phosphate solution obtained in Example 31 (B), 1.25 kgof a 40% colloidal silica solution and 1.32 kg of deionized water weremixed, and the mixture was sprayed-dried to obtain spherical solidparticles having an average particle size of 56 μm. The solid particleswere calcined in the same manner as in Example 31 (C) to obtain acatalyst composition (Catalyst No. 33).

EXAMPLES 33 TO 36 (C) Preparation of a catalyst compositions (CatalystsNos. 34 to 37)

The precursory oxide obtained in Example 31 (A) or the calcined oxidepowder obtained in Example 32 (A), the vanadyl phosphate solutionobtained in Example 31 (B) and the colloidal silica solution were mixedin the various proportions as shown in Table 12. The mixtures therebyobtained were respectively spray-dried to obtain spherical solidparticles, and the solid particles were calcined in the same manner asin Example 31 (C) to obtain catalyst compositions (Catalysts Nos. 34 to37)

The methods for the preparation of the Catalysts Nos. 31 to 37, theircompositions and the results obtained by measuring their physicalproperties are shown in Tables 12 and 13.

                  TABLE 12                                                        ______________________________________                                        Methods for the Preparation of the Catalysts                                                            Spray-drying                                                                           Oxide concen-                              Catalyst                                                                             A/B/C*   First     temperature                                                                            tration in                                 No.    (wt. %)  component (°C.)**                                                                         slurry (wt. %)                             ______________________________________                                        31     35/40/25 Precursory                                                                              130      20                                                         oxide                                                         32     35/40/25 Calcined  130      20                                                         oxide                                                         33      0/50/50 No addition                                                                             160      20                                         34     60/20/20 Precursory                                                                              170      45                                                         oxide                                                         35     45/35/20 Precursory                                                                              170      45                                                         oxide                                                         36     12/63/25 Calcined  150      20                                                         oxide                                                         37     35/40/25 Calcined  140      35                                                         oxide                                                         ______________________________________                                         *A/B/C: Dry weight ratio of First component/Second component/Third            component                                                                     **An average temperature of the gas in the chamber                       

                                      TABLE 13                                    __________________________________________________________________________    Compositions and Physical Properties of the Catalysts                                  Composition     Pore volume                                                                            Meso                                                 Content                                                                             P/V Specific                                                                            (ml/g)   pore                                        Exp.                                                                              Catalyst                                                                           of First                                                                            atomic                                                                            surface                                                                             37- 2000-                                                                              ratio                                       No. No.  component                                                                           ratio                                                                             area (m.sup.2 /g)                                                                   2000Å                                                                         20000Å                                                                         (%)*                                                                              Strength**                              __________________________________________________________________________    Exp.                                                                              31    35%  1.16                                                                              3.9   0.043                                                                             0.016                                                                              72  0.9                                     31                                                                            Exp.                                                                              32   35    1.16                                                                              5.0   0.097                                                                             0.016                                                                              55  1.2                                     32                                                                            Comp.                                                                             33    0    1.27                                                                              9.9   0.070                                                                             0.006                                                                              67  0.40                                    Exp.                                                                          Exp.                                                                              34   60    1.08                                                                              32.0  0.209                                                                             0.026                                                                              40  31                                      33                                                                            Exp.                                                                              35   45    1.08                                                                              21.3  0.158                                                                             0.036                                                                              30  19                                      34                                                                            Exp.                                                                              36   12    1.16                                                                              5.0   0.120                                                                             0.006                                                                              52  2.0                                     35                                                                            Exp.                                                                              37   35    1.16                                                                              8.0   0.020                                                                             0.028                                                                              34  4.2                                     36                                                                            __________________________________________________________________________     *The ratio (%) of the pores having a pore radius within a range of from       100 to 350 Å in the total pores having a pore radius within a range o     from 37 to 2000 Å                                                         **The catalyst composition particles were driven together with air from       high velocity nozzles to collide against a metal plate, whereby the           destruction loss (%) of the catalyst is measured. The smaller the value,      the greater the strength.                                                

EXAMPLES 31 TO 36 AND COMPARATIVE EXAMPLE 1 (D) Reaction Example 1

With use of an air-gas mixture containing 3% by volume of n-butane, anactivity test was conducted by means of a small fluidized bed reactormade of hard glass with 20 ml of the catalyst composition and at a flowrate of GHSV 500. The reaction product was absorbed in water, and thequantitative analysis was carried out by potentiometric titration of theaqueous solution and gas chromatography of the exhaust gas. The resultsthereby obtained are shown in Table 14.

                  TABLE 14                                                        ______________________________________                                                        Reaction   Conversion                                                                            Yield of maleic                            Example                                                                              Catalyst temperature                                                                              of n-butane                                                                           anhydride                                  No.    No.      (°C.)                                                                             (%)     (%)                                        ______________________________________                                        Exp. 31                                                                              31       440        86.1    45.4                                       Exp. 32                                                                              32       425        86.5    49.5                                       Comp.  33       500        68.5    21.4                                       Exp. 1                                                                        Exp. 33                                                                              34       440        78.0    34.1                                       Exp. 34                                                                              35       430        77.0    36.5                                       Exp. 35                                                                              36       470        68.5    32.0                                       Exp. 36                                                                              37       450        85.0    44.5                                       ______________________________________                                    

(D) Reaction Example 2

Each of the Catalysts 32 and 34 and the calcined oxide powder of Example32 (C) was pulverized in a mortar, and pelletized into pellets of 7 mmφ×2 mm H by a pelletizer. The pellets were crushed and sieved to obtainparticles having a particle size within a range of from 14 to 24 mesh.In a micro reactor made of hard glass and having a diameter of 6 mm, 1ml of the particles were filled, an air-gas mixture containing 1.5% byvolume of n-butane was reacted at a flow rate of GHSV 2000. The productwas analyzed by gas chromatography. The catalytic activities werecompared. The results thereby obtained are shown in Table 15.

                  TABLE 15                                                        ______________________________________                                               Content of                     Yield of                                Catalyst                                                                             First compo-                                                                             Reaction   Conversion                                                                            maleic an-                               No.    nent       Temperature                                                                              of n-butane                                                                           hydride                                  ______________________________________                                        32      35%          462° C.                                                                         94.0%   54.5%                                   34     60         455        84.3    40.5                                     Calcined                                                                             100        460        92.9    55.0                                     oxide                                                                         ______________________________________                                    

From the above Table, it is evident that the catalytic activities cannot be determined merely by the ratio of the first component. Especiallywhen the ratio of the pores having a pore radius within a range of from100 to 350 Å is low, the catalyst tends to have a lower catalyticactivity.

(D) Reaction Example 3

Into the small fluidized bed reactor used in the above Reaction Example1, 20 ml of the Catalyst 32 was filled, and an air-gas mixturecontaining 3% by volume of n-butane and 0.8% by volume of 1-butene wasintroduced and reacted. At a flow rate of GHSV 500 and at a reactiontemperature of 440° C., the conversion of n-butane was 81.3% and theconversion of 1-butene was 100%, and the yield of maleic anhydriderelative to the total hydrocarbons was 45.6%.

Further, the gas mixture introduced into the reactor was changed to anair-gas mixture containing 3% by volume of 1-butene and the reaction wascarried out in the same manner. At the flow rate of GHSV 500 and at thereaction temperature of 400° C., the conversion of 1-butene was 100%,and the yield of maleic anhydride was 56.0%.

We claim:
 1. A process for preparing an oxidation catalyst composition,which comprises:(a) mixing, as the first component (i), at least onecrystalline composite oxide selected from the group consisting of (1) acrystalline composite oxide containing vanadium and phosphorus havingthe characteristic X-ray diffraction peaks identified in Table A and (2)a crystalline composite oxide containing vanadium and phosporus havingthe characteristic X-ray diffraction peaks identified in Table B,

                  TABLE A                                                         ______________________________________                                        X-ray diffraction peaks                                                       (Anticathode: Cu--K.sub.α)                                              2θ (±0.2°)                                                    ______________________________________                                        15.7°                                                                  19.6°                                                                  24.2°                                                                  27.1°                                                                  28.8°                                                                  30.4°                                                                  ______________________________________                                    

                  TABLE B                                                         ______________________________________                                        X-ray diffraction peaks                                                       (Anticathode: Cu--K.sub.α)                                              2θ (±0.2°)                                                    ______________________________________                                        14.2°                                                                  15.7°                                                                  18.5°                                                                  23.0°                                                                  28.4°                                                                  30.0°                                                                  33.7°                                                                  36.8°                                                                  ______________________________________                                    

as the second component (ii), an aqueous solution containing vanadiumand phosphorus, and, as the third component (iii), silica sol, therebyforming an aqueous slurry of said components; (b) spray-drying theslurry; and (c) calcining the solid particles thereby obtained.
 2. Theprocess according to claim 1 wherein the second component is an aqueoussolution containing vanadyl phosphate.
 3. The process according to claim1 wherein the mixing ratio by dry weight of the first, second and thirdcomponents is within a range of:first component:second component:thirdcomponent=1:0.1-7:0.05-4.
 4. The process according to claim 1 whereinthe atomic ratio of phosphorus to vanadium in the oxidation catalystcomposition obtained is within a range of from 0.8 to 1.5.
 5. Theprocess according to claim 1 wherein the content of the first componentin the oxidation catalyst composition obtained is within a range of from15 to 80% by weight.
 6. The process according to claim 1 wherein thefirst component is a crystalline composite oxide containing vanadium andphosphorus having the characteristic X-ray diffraction peaks asidentified in Table A.
 7. The process according to claim 6 wherein thefirst component is a crystalline composite oxide present in an aqueousslurry obtained by the hydrothermal treatment of an aqueous solutioncontaining pentavalent phosphorus and tetravalent vanadium at atemperature of from 110° to 250° C.
 8. The process according to claim 6wherein the first component is a crystalline composite oxide obtained byspray-drying an aqueous slurry obtained by hydrothermal treatment of anaqueous solution containing pentavalent phosphorus and tetravalentvanadium at a temperature of from 110° to 250° C.
 9. The processaccording to claim 6 wherein the first component is a crystallinecomposite oxide obtained by solid-liquid separation of an aqueous slurryobtained by the hydrothermal treatment of an aqueous solution containingpentavalent phosphorus and tetravalent vanadium at a temperature of from110° to 250° C., and the second component is an aqueous solutionobtained by adding and dissolving phosphoric acid, a reducing agent andvanadium pentoxide in an aqueous solution obtained by the solid-liquidseparation.
 10. The process according to claim 6 wherein the secondcomponent is an aqueous solution containing vanadyl phosphate.
 11. Theprocess according to claim 1 wherein the first component is acrystalline composite oxide containing vanadium and phosphorus andshowing the characteristic X-ray diffraction peaks as identified inTable B.
 12. The process according to claim 11 wherein the firstcomponent is a crystalline composite oxide obtained by calcining solidparticles obtained by spray-drying an aqueous slurry obtained by thehydrothermal treatment of an aqueous solution containing pentavalentphosphorus and tetravalent vanadium at a temperature of from 110° to250° C.
 13. The process according to claim 11 wherein the firstcomponent is a crystalline composite oxide obtained by calcining solidparticles obtained by solid-liquid separation of an aqueous slurryobtained by the hydrothermal treatment of an aqueous solution containingpentavalent phosphorus and tetravalent vanadium at a temperature of from110° to 250° C., and the second component is an aqueous solutionobtained by adding and dissolving phosphoric acid, a reducing agent andvanadium pentoxide in an aqueous solution obtained by the solid-liquidseparation.
 14. The process according to claim 11 wherein the secondcomponent is an aqueous solution containing vanadyl phosphate.